MSVC: Process Makefile line continuations more like "make" does.

Unlike "make" itself, the MSVC build process recognized a continuation
even with whitespace after the backslash.  (Due to a typo, some code
sites accepted the letter "s" instead of whitespace).  Also, it would
consume any number of newlines following a single backslash.  This is
mere cleanup; those behaviors were unlikely to cause bugs.